Revenue - Schedule of Effect of ASC 606 on Financial Statements (Details) - USD ($) $ / shares in Units, $ in Millions		3 Months Ended						9 Months Ended
		Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Sep. 30, 2018		Sep. 30, 2017	Jan. 01, 2018	Dec. 31, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounts receivable, net		$ 2,132						$ 2,132			$ 2,555	$ 2,246
Prepaid and other current assets		547						547				430
Fixed assets, net		944						944				985
Other non-current assets		500						500				447
Deferred revenue and accrued expenses		1,465						1,465				1,711
Other current liabilities		758						758				804
Deferred tax liabilities		678						678				615
Provision for liabilities		585						585				558
Retained earnings	[1]	1,102						1,102				1,104
Revenue		1,859			$ 1,852			6,141		$ 6,124
Salaries and benefits		1,238			1,212			3,890		3,676
Depreciation		53			54			153		151
Income from operations		17			(26)			339		436
INCOME FROM OPERATIONS BEFORE INCOME TAXES		36			(35)			374		412
Benefit (provision) for income taxes		10			(19)			(42)		(73)
NET INCOME		46	$ 65	$ 221	(54)	$ 41	$ 352	332		339
Net income attributable to Willis Towers Watson		$ 44			$ (54)			$ 317		$ 323
Basic earnings per share		$ 0.34			$ (0.40)			$ 2.40		$ 2.38
Diluted earnings per share		$ 0.33			$ (0.40)			$ 2.39		$ 2.36
Net cash from operating activities								$ 716		$ 515
Capitalized software costs								(41)		$ (52)
Calculated under revenue guidance in Topic 606 [Member] | Accounting Standards Update 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounts receivable, net											2,555
Prepaid and other current assets											519
Fixed assets, net											902
Other non-current assets											486
Deferred revenue and accrued expenses											1,637
Deferred tax liabilities											714
Provision for liabilities											570
Retained earnings											1,421
Calculated under Revenue Guidance in Effect before Topic 606 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounts receivable, net												2,246
Prepaid and other current assets												430
Fixed assets, net												985
Other non-current assets												447
Deferred revenue and accrued expenses												1,711
Deferred tax liabilities												615
Provision for liabilities												558
Retained earnings												$ 1,104
Calculated under Revenue Guidance in Effect before Topic 606 [Member] | Accounting Standards Update 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounts receivable, net		$ 2,158						2,158
Prepaid and other current assets		478						478
Fixed assets, net		1,050						1,050
Other non-current assets		448						448
Deferred revenue and accrued expenses		1,546						1,546
Other current liabilities		827						827
Deferred tax liabilities		579						579
Provision for liabilities		574						574
Retained earnings		1,073						1,073
Revenue		1,901						6,474
Salaries and benefits		1,225						3,849
Depreciation		59						169
Income from operations		66						697
INCOME FROM OPERATIONS BEFORE INCOME TAXES		85						732
Benefit (provision) for income taxes								(111)
NET INCOME		85						621
Net income attributable to Willis Towers Watson		$ 83						$ 606
Basic earnings per share		$ 0.63						$ 4.59
Diluted earnings per share		$ 0.63						$ 4.58
Net cash from operating activities								$ 753
Capitalized software costs								(78)
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | Accounting Standards Update 2014-09 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Accounts receivable, net	[2]	$ (26)						(26)			309
Prepaid and other current assets	[3]	69						69			89
Fixed assets, net	[4]	(106)						(106)			(83)
Other non-current assets	[4]	52						52			39
Deferred revenue and accrued expenses	[5]	(81)						(81)			(74)
Other current liabilities	[6]	(69)						(69)
Deferred tax liabilities	[6]	99						99			99
Provision for liabilities	[7]	11						11			12
Retained earnings	[8]	29						29			$ 317
Revenue	[9],[10],[11],[12]	(42)						(333)
Salaries and benefits	[13],[14]	13						41
Depreciation	[14]	(6)						(16)
Income from operations		(49)						(358)
INCOME FROM OPERATIONS BEFORE INCOME TAXES		(49)						(358)
Benefit (provision) for income taxes		10						69	[15]
NET INCOME		(39)						(289)
Net income attributable to Willis Towers Watson		$ (39)						$ (289)
Basic earnings per share		$ (0.29)						$ (2.19)
Diluted earnings per share		$ (0.30)						$ (2.19)
Net cash from operating activities	[16]							$ (37)
Capitalized software costs	[16]							$ 37

[1] Equity includes (a) Ordinary shares $0.000304635 nominal value; Authorized 1,510,003,775; Issued 129,912,918 (2018) and 132,139,581 (2017); Outstanding 129,912,918 (2018) and 132,139,581 (2017); (b) Ordinary shares, €1 nominal value; Authorized and Issued 40,000 shares in 2018 and 2017; and (c) Preference shares, $0.000115 nominal value; Authorized 1,000,000,000 and Issued none in 2018 and 2017.

[2] Accounts receivable, net, now includes receivables that have been billed, not yet billed and short-term contract assets. This adjustment is the result of the cumulative adjustments to revenue that have not yet been collected from our customers, but are expected to be collected within the next twelve months. The most significant increases to this balance result from revenue acceleration under ASC 606 for Medicare and proportional treaty broking commissions.

[3] Prepaid and other current assets include the impact of costs deferred in connection with our broking pre-placement activities. These costs are being deferred while the related pre-placement work is performed, and amortized as the related revenue is recognized, typically upon policy inception. Since the amortization period associated with these fulfillment costs is less than one year, these deferred costs have been classified as a current asset.

[4] Prior to the adoption of ASC 606, costs that we deferred related to certain system implementation activities had been included in fixed assets, net. These costs, adjusted based on the guidance in ASC 606, have now been included in other non-current assets. Additionally we have included less significant impacts of adjustments to deferred tax assets and have classified non-current contract assets within non-current assets.

[5] Deferred revenue has been adjusted primarily to reflect revenue acceleration in our Medicare broking business. Additional adjustments were included to accelerate the license component of certain software arrangements and to net deferred revenue with contract assets.

[6] Other current liabilities, which includes current taxes payable, and deferred tax liabilities, have been adjusted for the tax effects of the individual changes resulting from the adoption of ASC 606. The income tax expense was calculated based on the U.S. and foreign statutory rates applicable to adjustments made. Where applicable, a U.S. statutory rate of 21% was used.

[7] Provision for liabilities has been adjusted for additional reserves for long-term post-placement obligations in our broking business.

[8] Retained earnings has been adjusted for the net impact of the adoption of ASC 606. See the discussion of the significant pre-tax changes by revenue stream in the following section.

[9] Health and benefits broking — Revenue for certain Health and Benefits broking arrangements, in our Human Capital and Benefits segment, will now be recognized evenly over the year to reflect the nature of the ongoing obligations to our customers as well as receipt of the monthly commissions. These contracts are monthly or annual in nature, and are considered complete as of the transition date. Therefore, no retained earnings adjustment is required. The total changes to revenue as a result of this accounting change for the three and nine months ended September 30, 2018 was an increase of $39 million and a decrease of $116 million, respectively.

[10] Medicare broking — The majority of revenue recognition for this offering, within our Individual Marketplace business, has moved from monthly ratable recognition over the policy period, to recognition upon placement of the policy. Consequently, the Company will now recognize approximately two-thirds of one calendar year of expected commissions during its fourth quarter of the preceding calendar year. The remainder of the revenue is recognized consistently with methods used prior to the adoption of ASC 606. Therefore, at the adoption date, we have reflected a $271 million pre-tax increase to retained earnings for the portion of the revenue that would otherwise have been recognized during our 2018 calendar year since our earnings process was largely completed during the fourth quarter of 2017. Additionally, we have reflected a $40 million pretax adjustment to increase retained earnings related to previously deferred contingent revenue from placements made prior to 2018 because the earnings process was complete under ASC 606. During the three and nine months ended September 30, 2018, the accounting for this revenue stream under ASC 606 represented a reduction of revenue from ASC 605, Revenue Recognition (‘ASC 605’) accounting methods of $74 million and $225 million, respectively.

[11] Other adjustments — Certain other revenue changes with individually less significant adjustments were made to retained earnings as of the adoption date totaling a net $28 million. The cumulative changes to revenue for the three and nine months ended September 30, 2018 for other revenue streams not discussed above resulting from the ASC 606 adoption was an increase of $4 million and a decrease of $10 million, respectively.

[12] Proportional treaty reinsurance broking — The revenue recognition for proportional treaty reinsurance broking commissions, within our Investment, Risk and Reinsurance segment, has moved from recognition upon the receipt of the monthly or quarterly treaty statements from the ceding insurance carriers, to the recognition of an estimate of expected commissions upon the policy effective date. Since the majority of revenue recognized historically based on these monthly or quarterly statements was received over a two-year period, we reflected a $50 million pretax increase to retained earnings at the adoption date for the portion of revenue that would otherwise have been recognized during our 2018 calendar year related to policies effective in 2017 or prior years. For the three and nine months ended September 30, 2018, this accounting change resulted in a revenue decrease of $11 million and a revenue increase of $18 million, respectively, related to this adjustment

[13] Other cost adjustments — This guidance now applies to our broking arrangements and certain consulting engagements. While the costs deferred for our broking arrangements will typically be amortized within one year, costs now deferred related to certain consulting arrangements will be amortized over a longer term. We have increased pre-tax retained earnings by $75 million primarily to reflect the total changes to contract costs as of the adoption date. For the three and nine months ended September 30, 2018, these changes resulted in expenses increasing by $1 million and $15 million, respectively.

[14] System implementation activities — For those portions of the business that previously deferred costs, the length of time over which we amortize those costs will extend to a longer estimated contract term. For 2017 and prior years, these costs were amortized over a typical period of 3-5 years in accordance with the initial stated terms of the customer agreements. Additionally, the composition of deferred costs has been adjusted to reflect the guidance in ASC 606. A reduction adjustment to retained earnings of $46 million was recorded on the adoption date to reflect these changes. Further, the amortization of the costs are no longer classified as depreciation expense, but rather included in salaries and benefits. These adjustments resulted in increases in expenses of $6 million and $10 million for the three and nine months ended September 30, 2018, respectively.

[15] The provision for income taxes for the three and nine months ended September 30, 2018 was $10 million and $69 million, respectively, lower than our provision on an ASC 605 basis. The income tax expense was calculated based on the U.S. and foreign statutory rates applicable to adjustments made. Where applicable, a U.S. statutory rate of 21% was used. There was a $101 million net tax reduction to retained earnings upon adoption of ASC 606.

[16] As part of the changes in accounting for deferred costs, amounts capitalized relating to system implementation activities are now classified as operating cash outflows. Prior to 2018, those costs capitalized under previous guidance were included in the Capitalized software costs as an investing cash outflow.